This invention relates to the field of optical fibers. More specifically, to the field of devices for removing protective layers from fiber optic cables. Even more specifically, to the field of devices for removing protective layers from fiber optic cables through the use of laser beams.
Optical fibers (also simply referred to as xe2x80x9cfibersxe2x80x9d in this patent) are made of a core, a cladding layer and one or multiple protective coatings or layers. The core material is typically glass or fused silica and the core diameter ranges from a few microns for single mode fibers to a few hundred microns for multi-mode fibers. A cladding material of lower index of refraction such as glass, fused silica or sometimes plastic surrounds the core. The core and cladding material may each be doped to modify the index of refraction of the material or for some other specialized applications such as fiber amplifier and Bragg grating fibers. Finally the fiber is coated with one or more protective layers such as acrylate, silicon acrylate or other materials. These protective layers increase the fiber""s mechanical strength and protect it from physical and environmental damage such as that caused by moisture.
Many fiber optics applications require removal of the protective layer. In one example, fibers are stripped at the ends of the fibers to allow fusion splicing of similar or different fiber types, as in the splicing of a specialty fiber such as an erbium doped fiber to a transmission fiber. Fibers are also stripped at the ends to connect an opto-electronic module such as a transmitter, receiver, transceiver, repeater, regenerator, coupler, or wavelength division multiplexer. Any of these modules may require the connection to be hermetically sealed if it may be exposed to harsh environmental conditions. For instance, a wavelength division demultiplexer may be located on the roof or the wall of a building. Fibers with protective coatings such as acrylate can not be directly used in hermetic seals. Polymer to fused silica interfaces and polymer to metal interfaces do not provide true hermetic seals. These interfaces are loosely bonded and susceptible to leaks and degradations. Most polymers are too permeable and degrade and outgas over time. For a true hermetic seal, the fibers must first be stripped of their protective coatings. The naked fused silica is then metalized by evaporation, sputtering, plating or other appropriate metalization means. The metalized fibers are sealed by soldering, which supplies a true hermetic seal between the metalized fibers and the metal case of the module.
The fiber stripping for these purposes is now usually done mechanically with a wire-stripping type tool or by dipping the fiber in the proper chemical etching bath. These methods have several shortcomings. Thermo-mechanical stripping is limited to single fibers and not applicable to ribbons. Furthermore, it is fairly unreliable as the blades wear with usage resulting in incomplete stripping or mechanical damage to the fiber itself. Chemical stripping requires large amounts of highly corrosive acids, which are environmentally unfriendly. Chemical stripping is a fairly slow process, which takes a few minutes including strip, wash and dry. It may leave residues that lead to poor adhesion of the metal film deposited on the fiber. Chemical stripping is difficult to automate. Furthermore, for mid-span stripping, because there is a minimum of fiber bend radius, when dipping the fiber in the chemical etch bath, the strip length is limited to around 15 mm and longer. Shorter strip length is not possible with chemical stripping.
Stripping of fibers in mid-span may also be required. This stripping is much more difficult to achieve than stripping the end of a fiber. For example, to hermetically seal a module with fibers feeding through the box or the connector, it is necessary to first strip the protective coating in the region where the fibers feed through the module enclosure. As described above, stripped fibers provide much better bonding surfaces for the metallized sealant than the protective layer surrounding the fibers. Fibers may also be stripped as a precursor step to writing Bragg gratings. In addition, if the optical fibers are assembled into flat ribbon cables, stripping of the protective layer from a mid-span section of the cable assembly is very difficult with conventional means such as acid bath or thermo-mechanical stripping.
Lasers have been used widely for stripping insulating layers of conventional copper wires. A laser beam is typically incident on the wire and the insulating layer is removed all the way through to the copper core on the side of the wire which is illuminated by the laser beam. The wire or the laser beam is then rotated to remove the protective layer from the other sides of the wire. In that application, the laser energy at a wide range of levels easily ablates the insulator and leaves the copper undamaged. Note that the removal process is very nonuniform because the insulating layer is removed completely from one side, but is left on the other sides of the fibers. Because the fused silica in the fiber form have optical and mechanical characteristics unlike copper, fibers are easily damaged with a similar non-uniform laser process, at laser ablation levels required to remove the protective coatings. Furthermore, as the fused silica transmits some of the laser energy through the fiber, this energy is focused by the fiber itself thus increasing the energy density of the laser beam inside the fiber and outside the fiber on the side of the fiber opposite the illuminating laser beam (see FIG. 1). Therefore, using lasers for stripping insulation is a much more complex operation with fiber cables than with copper wires. The process of stripping protective layers from fiber optic cable requires a much more xe2x80x9cgentlexe2x80x9d stripping process than the non-uniform method used for copper wires.
Moreover, for many applications, even more careful control of the laser energy or wavelength delivered by the optical system is required to strip the protective layer without affecting the index of refraction of the fiber core and cladding. For instance, in Bragg grating fibers, the core contains a dopant such as Germanium or the fiber may be loaded with Hydrogen to allow the writing of the grating on the fiber with UV light exposure. The core of this fiber is sensitive to UV light, and therefore, its index changes with UV exposure.
This invention results from the realization that a laser can remove the protective layer from the fiber core and cladding more effectively and more reliably than chemical or mechanical means, without significantly damaging the optical and mechanical properties of the fiber and without leaving an excessive amount of residual ablation debris on the fiber while providing careful control of the laser energy. This method thereby allows users to safely strip protective layers off sensitive fibers, such as fibers used for fiber Bragg gratings (FBG).
It is therefore an object of this invention that the protective layer of an optical fiber can be removed with minimum degradation of the optical and mechanical properties of the fiber core and cladding materials.
It is a further object of this invention to provide a method for removal of a protective layer from the fiber core and cladding that is more reliable than removal by mechanical means, particularly when mid-span stripping.
It is a further object of this invention to provide a means for the removal of protective layers of ribbon cable.
It is a further object of this invention that the removal of the protective layers can be performed by a dry, non-contact, reliable, and environmentally friendly method.
It is a further object of this invention that the fiber will not be substantially damaged by the removal of the protective layer.
It is a further object of this invention that unacceptable scoring will not be left on the fiber by removal of the protective layer.
It is a further object of this invention that removal of the protective layer will not leave an excessive amount of residual ablation debris on the fiber.
It is a further object of this invention that the laser energy can be sufficiently controlled to allow effective removal of the protective layer from more sensitive fibers, such as Bragg grating fibers.
It is a further object of this invention that removal of the protective layer from fiber in this manner will leave the fiber in proper condition for metalizing and then sealing the fiber, creating an hermetic seal.